The present invention relates generally to the non-contact repair of micro-circuits, and in particular to the repair of active matrix liquid crystal display panels.
During the manufacturing of liquid crystal (LC) displays, large clear plates of thin glass are used as a substrate for the deposition of thin film transistor (TFT) arrays. Usually, several independent TFT arrays are contained within one glass substrate plate and are often referred to as TFT panels. Alternatively, an active matrix LCD, or AMCLD, covers the class of displays utilizing a transistor or diode at every subpixel, and therefore encompasses TFT devices, such glass substrate plates may also be referred to as AMLCD panels. Flat panel displays may also be fabricated using any of the OLED technologies and though typically fabricated on glass, may also be fabricated on plastic substrate plates.
TFT pattern deposition is performed in a multitude of stages where in each stage, a particular material (such as a metal, indium tin oxide (ITO), crystalline silicon, amorphous silicon, etc.) is deposited on top of a previous layer (or glass) in conformity with a predetermined pattern. Each stage typically includes a number of steps such as deposition, masking, etching, stripping, etc.
During each of these stages and at various steps within each stage, many production defects may occur that may affect the electrical and/or optical performance of the final LCD product. Such defects include but are not limited to metal protrusion 110 into ITO 112, ITO protrusion 114 into metal 116, a so-called mouse bite 118, an open circuit 120, a short 122 in a transistor 124, and a foreign particle 126, as shown in FIG. 1. Other defects include mask problems, over or under etching, etc.
Even though the TFT deposition processes are tightly controlled, defect occurrence is unavoidable. This limits the product yield and adversely effects production costs. Typically, the TFT arrays are inspected using one or multiple Automated Optical Inspection (AOI) system(s) following critical deposition stages and by an electro-optical inspection machine, also referred to as array tester or array checker (AC) to test the finished TFT arrays. Commonly AOI and AC systems provide defect coordinates; they do not provide high resolution images required to classify defects as killer, reparable or just imperfections not affecting the TFT array performance (so called process defects). The defect coordinate information is passed to a TFT array repair tool, also referred to as array saver (AS), and such classification is conventionally done manually by the TFT array repair machine operator.
The average number of defects per plate may vary from one TFT array manufacturer to another and from one manufacturing plant to another. Typically, the defect review and repair capacity within the TFT array fabrication line is sized to process 300 to 400 defects per 7th generation plates. Typically 5 to 10% of defects per plate may require repair.
Since the TFT array features are typically very small (sub-pixel size may be 80 micrometer by 240 micrometer and up to 216 micrometer by 648 micrometer for large 40 inch LCD televisions made from 7th generation plates), the array repair tool includes a microscope to perform a defect review to decide whether the defect is repairable. The microscope field of view is small, ranging from 100 micrometer by 100 micrometer to 2 millimeter by 2 millimeter, relative to the plate size, which is typically 2.1 meter by 2.4 meter. The microscope is installed on a precision XY stage so that it can be dispatched from one defect to another. The defect coordinates are known from inspections carried out earlier by AOI and AC inspection systems. The glass plate remains immobilized under the XY stage by means of a vacuum chuck during the defect review and repair. Following the review, the repairable defects are typically treated by means of laser trimming, laser welding or by bridging open line.
The above series of general events is typical of all array repair tools. However, because the number, type, locations, size, degree of defects often vary from panel to panel, a means to pass judgment is required at almost all of the tool steps following capture of the defect images—for example, whether an image is truly a defect rather than nuisance, what kind of defect has been found, whether or not a specific defect needs repair, what kind of repair is required, what repair parameters are needed, what is the next defect to be repaired, and so forth. Many repair tools combine tool operation with human operator judgment and intervention to identify, classify, and then repair such defects.
FIG. 2 shows a defect repair example in cross section. Several typical layers are shown: a passivation layer 210 may cover the metal circuitry 212 and both lie on top of substrate 214. Metal protrusion defect 110 is shown in FIG. 2A (see FIG. 1 for top view). In this example, after identifying and classifying the defect 110, a repair recipe is created, and then executed to remove the protrusion using laser 220, as shown in FIG. 2B. Precision laser abalation or laser micro-machining requires matching the laser properties (wavelength, pulsewidth, energy, etc.) to the material being removed. Material removal can be a relatively straightforward process, using typical laser cutting or ablation techniques in which first the most appropriate wavelength is selected followed by careful optimization of the laser energy and process.
However, in some cases the material to be removed does not abalate well and excessive debris is generated in the process. In other cases, a thin layer of one material needs to be cleanly removed without damaging an underlying layer which may respond differently to the laser wavelength due to having different optical properties. For such thin layers, the laser fluence (energy per area) must be uniform over the beam cross-section so as not to damage underlayers inadvertently because of “hot” spots within the beam cross-section, or to leave excess material because of having too little energy density. Multiple passes to clean up the debris are sometimes used, although often this leads to damage to the substrate or other layers underneath and longer tact times.
FIG. 3A shows a layer to be removed 310 on top of layer 312, which must not be damaged, both of which lie on substrate 314. FIG. 3B shows application of a laser beam 320 having wavelength λ1 to layer 310. FIG. 3C illustrates the results after attempting to ablate layer 310 with laser beam 320 at a fluence that does not damage underlayer 312. As shown, the material making up layer 310 may generate debris 330, which must be removed before continuing fabrication of the circuitry.
Ensuring that no debris remains may be addressed in several ways. One conventional method is to apply a higher fluence laser beam 320, as shown in FIG. 3D, so that no debris is generated, but this method increases the likelihood of damage to the underlayer 312, which may be partially ablated due to the higher fluence, as shown in FIG. 3E. Another prior art method allows generation of debris as in the steps illustrated by FIGS. 3A, 3B and 3C, but then introduces an additional step in which a laser of different wavelength λ2 322 is directed at the debris 330 (FIG. 3F). In this case, the debris 330 and the underlayer 312 may have similar optical properties, and therefore the underlayer may still be at risk for damage, since it will absorb λ2 as well (FIG. 3E). In the case in which the debris and underlayer have different optical properties, removal of the debris in a subsequent step using wavelength λ2 requires careful control of the laser fluence, since too high a fluence may still damage the underlayer, while too low a fluence may be insufficient to ablate the debris. Since debris may be of different size and volume, the laser fluence will need to be adjusted for each case. That is, the process window to achieve clean cuts without residual debris may be very narrow making automation of such a debris removal process difficult.